The correlative studies of 3H isoleucine uptake in pia arachnoid explants and in the fibroblastic culture of skin have shown that the uptake is saturable and could be cross inhibited by L leucine and cycloleucine in both types of cultures. However, the pia arachnoid was found to be different from the fibroblasts since it 1. couldn't accumulate the amino acid as did the fibroblasts and 2. couldn't be cross inhibited by D leucine, thus displaying stereospecificity for the L form of the amino acid. Therefore, the pia arachnoid may represent a site for regulation of amino acid transport in the central nervous system.